Enable Travis CI

Travis CI was developed for Ruby projects, so the configuration is arguably the easiest. Travic would run the `rake` command to test, but I thought it would be better to specify the full command for documentation's sake. The Gemfile was added because Travis will try to `bundle` the dependencies, which relies on a Gemfile. I tried to follow the [best practices](http://stackoverflow.com/questions/11301000/why-use-gemspec-gemfile-when-checking-for-dependencies).

For more, see the [Travis CI Getting Started Guide](http://about.travis-ci.org/docs/user/getting-started/) and [Ruby docs](http://about.travis-ci.org/docs/user/languages/ruby/).